2,542 research outputs found
A note on cosmological Levi-Civita spacetimes in higher dimensions
We find a class of solutions to cosmological Einstein equations that
generalizes the four dimensional cylindrically symmetric spacetimes to higher
dimensions. The AdS soliton is a special member of this class with a unique
singularity structure.Comment: 3 pages; version to appear in Phys. Rev.
Gravitational charges of transverse asymptotically AdS spacetimes
Using Killing-Yano symmetries, we construct conserved charges of spacetimes
that asymptotically approach to the flat or Anti-de Sitter spaces only in
certain directions. In D dimensions, this allows one to define gravitational
charges (such as mass and angular momenta densities) of p-dimensional
branes/solitons or any other extended objects that curve the transverse space
into an asymptotically flat or AdS one. Our construction answers the question
of what kind of charges the antisymmetric Killing-Yano tensors lead to.Comment: 11 pages, no figures, REVTeX 4; version 2: important corrections
made; version 3: one new paragraph and 2 references added, accepted for
publication in PR
Cooperative Binning for Semi-deterministic Channels with Non-causal State Information
The capacity of the semi-deterministic relay channel (SD-RC) with non-causal
channel state information (CSI) only at the encoder and decoder is
characterized. The capacity is achieved by a scheme based on
cooperative-bin-forward. This scheme allows cooperation between the transmitter
and the relay without the need to decode a part of the message by the relay.
The transmission is divided into blocks and each deterministic output of the
channel (observed by the relay) is mapped to a bin. The bin index is used by
the encoder and the relay to choose the cooperation codeword in the next
transmission block. In causal settings the cooperation is independent of the
state. In \emph{non-causal} settings dependency between the relay's
transmission and the state can increase the transmission rates. The encoder
implicitly conveys partial state information to the relay. In particular, it
uses the states of the next block and selects a cooperation codeword
accordingly and the relay transmission depends on the cooperation codeword and
therefore also on the states. We also consider the multiple access channel with
partial cribbing as a semi-deterministic channel. The capacity region of this
channel with non-causal CSI is achieved by the new scheme. Examining the result
in several cases, we introduce a new problem of a point-to-point (PTP) channel
where the state is provided to the transmitter by a state encoder.
Interestingly, even though the CSI is also available at the receiver, we
provide an example which shows that the capacity with non-causal CSI at the
state encoder is strictly larger than the capacity with causal CSI
Noncontact lateral-force gradient measurement on Si(111)-7×7 surface with small-amplitude off-resonance atomic force microscopy
In this work, the authors report on a quantitative investigation of lateral-force gradient and lateral force between a tungsten tip and Si(111)-(7×7) surface using combined noncontact lateral-force microscopy and scanning tunneling microscopy. Simultaneous lateral-force gradient and scanning tunneling microscopy images of single and multiatomic step are obtained. In our measurement, tunnel current is used as feedback. The lateral-stiffness contrast has been observed to be 2.5 N/m at a single atomic step, in contrast to 13 N/m at a multiatomic step on Si (111) surface. They also carried out a series of lateral stiffness-distance spectroscopy, which show a sharp increase in tip-surface interaction stiffness as the sample is approached toward the surface
Spherically symmetric solutions of Einstein + non-polynomial gravities
We obtain the static spherically symmetric solutions of a class of
gravitational models whose additions to the General Relativity (GR) action
forbid Ricci-flat, in particular, Schwarzschild geometries. These theories are
selected to maintain the (first) derivative order of the Einstein equations in
Schwarzschild gauge. Generically, the solutions exhibit both horizons and a
singularity at the origin, except for one model that forbids spherical symmetry
altogether. Extensions to arbitrary dimension with a cosmological constant,
Maxwell source and Gauss-Bonnet terms are also considered.Comment: 6 pages, no figures, REVTeX
Illumination and annealing characteristics of two-dimensional electron gas systems in metal-organic vapor-phase epitaxy grown AlGaN/AlN/GaN heterostructures
We studied the persistent photoconductivity (PPC) effect in AlGaN/AlN/GaN
heterostructures with two different Al-compositions (x=0.15 and x=0.25). The
two-dimensional electron gas formed at the AlN/GaN heterointerface was
characterized by Shubnikov-de Haas and Hall measurements. Using optical
illumination, we were able to increase the carrier density of the
Al0.15Ga0.85N/AlN/GaN sample from 1.6x10^{12} cm^{-2} to 5.9x1012 cm^{-2},
while the electron mobility was enhanced from 9540 cm2/Vs to 21400 cm2/Vs at T
= 1.6 K. The persistent photocurrent in both samples exhibited a strong
dependence on illumination wavelength, being highest close to the bandgap and
decreasing at longer wavelengths. The PPC effect became fairly weak for
illumination wavelengths longer than 530 nm and showed a more complex response
with an initial negative photoconductivity in the infrared region of the
spectrum (>700 nm). The maximum PPC-efficiency for 390 nm illumination was
0.011% and 0.005% for Al0.25Ga0.75N/AlN/GaN and Al0.15Ga0.85N/AlN/GaN samples,
respectively. After illumination, the carrier density could be reduced by
annealing the sample. Annealing characteristics of the PPC effect were studied
in the 20-280 K temperature range. We found that annealing at 280 K was not
sufficient for full recovery of the carrier density. In fact, the PPC effect
occurs in these samples even at room temperature. Comparing the measurement
results of two samples, the Al0.25Ga0.75N/AlN/GaN sample had a larger response
to illumination and displayed a smaller recovery with thermal annealing. This
result suggests that the energy scales of the defect configuration-coordinate
diagrams for these samples are different, depending on their Al-composition.Comment: 27 pages, 8 figure
Cylindrically symmetric, static strings with a cosmological constant in Brans-Dicke theory
The static, cylindrically symmetric vacuum solutions with a cosmological
constant in the framework of the Brans-Dicke theory are investigated. Some of
these solutions admitting Lorentz boost invariance along the symmetry axis
correspond to local, straight cosmic strings with a cosmological constant. Some
physical properties of such solutions are studied. These strings apply
attractive or repulsive forces on the test particles. A smooth matching is also
performed with a recently introduced interior thick string solution with a
cosmological constant.Comment: 8 pages, Revtex; Published versio
Topologically massive gravity as a Pais-Uhlenbeck oscillator
We give a detailed account of the free field spectrum and the Newtonian limit
of the linearized "massive" (Pauli-Fierz), "topologically massive"
(Einstein-Hilbert-Chern-Simons) gravity in 2+1 dimensions about a Minkowski
spacetime. For a certain ratio of the parameters, the linearized free theory is
Jordan-diagonalizable and reduces to a degenerate "Pais-Uhlenbeck" oscillator
which, despite being a higher derivative theory, is ghost-free.Comment: 9 pages, no figures, RevTEX4; version 2: a new paragraph and a
reference added to the Introduction, a new appendix added to review
Pais-Uhlenbeck oscillators; accepted for publication in Class. Quant. Gra
Hardal uçucu yağı ve ana bileşiğinin tek başına ve değiştirilmiş atmosfer uygulamaları ile kombinasyonun Tribolium confusum du Val., 1863 (Coleoptera: Tenebrionidae)'a karşı fümigant etkisi]
This study was carried out in 2017 in Entomology Laboratory of Kahramanmaraş Sütçü İmam University to determine fumigant toxicity of mustard essential oil and its main compound (allyl isothiocyanate) alone and in combination with high concentration (92%) of CO2 or N2 to all life stages of Tribolium confusum du Val., 1863 (Coleoptera: Tenebrionidae) was determined. Preliminary bioassay tests indicated that 10 µl/l of mustard essential oil and allyl isothiocyanate alone resulted in 100% mortality for all life stages of T. confusum without any necessity of CO2 and N2 combinations. Lethal concentration tests indicated that combinations of mustard essential oil or allyl isothiocyanate with 92% CO2 produced 1.8 to 7.3 times reductions in LC90 values for larvae, pupae and adults of T. confusum. Generally, the combinations of mustard essential oil or allyl isothiocyanate with 92% CO2 were more toxic to larvae, pupae and adults of T. confusum than those in combinations with 92% N2 as evidenced by significant decrements in their LC50 and LC90 values. It appears that high concentration of CO2 or N2 might have a synergistic effect on larvae, pupae and adults of T. confusum when exposed together with mustard essential oil or allyl isothiocyanate. In conclusion, this study indicates that combinations of mustard essential oil or its main compound, allyl isothiocyanate with modified atmospheres can be a potential alternative to the most commonly used commercial fumigants, methyl bromide and phosphine. © 2019 Entomological Society of Turkey. All rights reserved
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